Abstract

Discussion of a paper by C. F. Davidson (GeoScience Abstracts 1-3144). Some "criteria" indicating a nonplacer origin for uraniferous conglomerates are criticized. One such "criterion" based on the grain size of host rock sediments relative to the grain size of uraninite is misleading because Stokes' Law cannot be applied. Cases are known where radioactivity is higher in coarse clastic sediments than in adjacent fine-grained ones. Most quartz-pebble conglomerates in the Blind River (Ontario) area contain more Th than U. There is no positive correlation between pyrite content and U content. Pyrite may be of multigenetic origin. Available evidence suggests that most other mineral assemblages, including those containing U, are of placer origin.